1. Technical Field
The present disclosure relates to a reference signal generator circuit for an analog-to-digital converter, in particular of an acoustic transducer, for example a MEMS (microelectromechanical system) capacitive microphone, to which the ensuing description will make explicit reference without implying any loss of generality; the present disclosure moreover relates to a method for generating the reference signal.
2. Description of the Related Art
As is known, an acoustic transducer of a capacitive type, for example, a MEMS microphone, generally includes a mobile electrode, provided as diaphragm or membrane, set facing a fixed electrode, to provide the plates of a variable-capacitance detection capacitor. The mobile electrode is generally anchored by means of a perimetral portion thereof to a substrate, whilst a central portion thereof is free to move or bend in response to the pressure exerted by incident sound waves. The mobile electrode and the fixed electrode form a capacitor, and bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of the capacitor. In use, the variation of capacitance, which is a function of the acoustic signal to be detected, is transformed into an analog electrical signal that is supplied as output signal of the acoustic transducer.
The analog electrical signal is generally converted into a digital signal so as to be appropriately processed. The operation of conversion is performed by means of an analog-to-digital (A/D) converter and is based, as is known, upon the comparison of the analog electrical signal at an input to the A/D converter with a reference voltage signal VREF, generated by an appropriate circuit external to the A/D converter and supplied on an input terminal of the latter.
The resolution with which the analog-to-digital converter carries out the operation of conversion is strictly dependent upon the noise superimposed on the reference signal VREF. It is hence fundamental, in order to guarantee a high signal-to-noise ratio, to have available a reference voltage VREF with low noise.
To overcome the limitation, a circuit solution has been proposed, illustrated in FIG. 1, in which a lowpass filter 1, in RC configuration, is connected to an output of the reference signal generator circuit 2 via an input terminal 3 of its own, and to an input of the analog-to-digital converter 4 via an output terminal 5 of its own, and has the function of filtering the reference signal VREF so as to attenuate the noise components thereof.
In particular, the lowpass filter 1 is provided with a filter resistor 6, connected between the input terminal 3 and the output terminal 5, and a filter capacitor 8 connected between the output terminal 5 and a ground terminal GND.
It has, however, been shown that, in order for the action of lowpass filtering to be effective, it is convenient for the lowpass filter 1 to present a pole at a frequency lower than the audio band (indicatively included between 20 Hz and 20 kHz), preferably a frequency equal to or lower than 1 Hz.
For this purpose, filter capacitors 8 are generally used, which have a high value of capacitance (for example, in the 100 nF-10 μF range) and, typically, cannot be integrated, as described, for example, in US 2008/0224759.
Alternatively, it is possible to use extremely high values of resistance of the filter resistor 6, included, for example, between 100 GΩ and 100 TΩ.
As is known, since it is not feasible in the technology of integrated circuits to produce resistors with such high values of resistance, use of nonlinear devices able to provide the high values of resistance required has been proposed. For example, there has been proposed for this purpose the use of a pair of diodes in antiparallel configuration, which provide a resistance sufficiently high when there is a voltage drop thereon of contained value (depending upon the technology of fabrication of the diodes, for example less than 100 mV).
As illustrated in FIG. 2, the filter resistor 6 can hence be provided by a respective pair of diodes in antiparallel configuration.
In particular, the filter resistor 6 is provided by a first diode 6a, with its anode connected to the input terminal 3 and its cathode connected to the output terminal 5, and by a second diode 6b, with its anode connected to the output terminal 5 and its cathode connected to the input terminal 3.
The main problem of circuit architectures of the above sort is represented by the long start-up time required for supply of a stable reference signal VREF to the A/D converter 4, on account of the presence of the pair of diodes 6a, 6b connected in antiparallel configuration and of the high value of resistance provided thereby. The settling time of a configuration of this sort may be of minutes or even hours; before the end of the settling time, i.e., throughout the period of start-up of the circuit, proper functioning of the lowpass filter 1 cannot be guaranteed, just as likewise a stable reference voltage VREF cannot be guaranteed.
During the start-up time, there hence occurs inevitably an even marked degradation in the performance of the A/D converter and of the corresponding MEMS microphone.
Only at the end of the long start-up time, does the voltage on the output terminal 5 stabilize at the desired reference value.
Clearly, such long delay times cannot be for example accepted in the common situations of use of the MEMS microphone, when instead it is necessary to guarantee the nominal performance with extremely short delays, both upon switching-on of a generic electronic device incorporating the MEMS microphone and upon return from a so-called “power-down” condition (during which the device itself is partially turned off to provide a condition of energy saving).